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The light spectrum encompasses ultraviolet, visible, and infrared energy. Although some thermal effects may be obtained from these modalities, the primary benefits are derived from photochemical effects.

• Electromagnetic energy is the most abundant form of energy in the universe. The energy found in the electromagnetic spectrum, including radio waves, and x-rays, is categorized by the frequency and length of its wave (Appendix B). Light, a form of electromagnetic energy, has three general classifications: ultraviolet, visible, and infrared (Fig. 19-1). Energy having a wavelength greater than 780 nanometers • (nm) (the upper end of visible light) is infrared energy. The ultraviolet (UV) spectrum is located in the area below the range of visible light (380 nm).

Figure 19-1.

The Electromagnetic Spectrum. The visible portion of the light spectrum consists of energy having a wavelength of approximately 380 to 780 nm. Infrared light has a wavelength between 780 and 12,500 nm; energy with a wavelength between 180 and 400 nm is in the ultraviolet range. There is a slight amount of overlap between the visible light range and that of ultraviolet and infrared light.

Many therapeutic modalities presented in this text use energy within the light range of the electromagnetic spectrum, although the light energy may not be visible to humans. UV light is used for the treatment of certain skin conditions. Medical lasers produce beams of energy that can cause either tissue destruction or therapeutic effects within the tissues.

Therapeutic Lasers

Lasers produce highly refined, monochromatic • light in the ultraviolet, visible, or infrared range. Lasers, an acronym for Light Amplification by Stimulated Emission of Radiation, consist of highly organized light (photons) • that elicits physiological events in the tissues. Low-level laser therapy (LLLT) does not normally cause tissue destruction.198 The U.S. Food and Drug Administration (FDA) has approved LLLT for the treatment of carpal tunnel syndrome • and musculoskeletal shoulder and neck pain; however, clinically it is used for a wide range of conditions.

The energy produced by therapeutic lasers can have a wavelength between 650 and 1200 nanometers (nm).199 This range includes UV, visible, and infrared light on the electromagnetic spectrum. The frequency (wavelength) determines the color of the laser light. Frequency and wavelength, often used interchangeably, are inversely related to each other: as frequency increases, wavelength decreases (and vice versa). The photons emitted during LLLT activate certain skin receptors that stimulate or inhibit physiological events. These effects are caused by activation of chromophores, parts of a molecule (generally melanin • and hemoglobin) that absorb light having a specific color (wavelength). Because of the specificity of chromophores in absorbing light energy, wavelength is thought to determine which skin receptors are affected.200

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